10
Operating information
(continued)
49
Installation & Operation Manual
Cascade
When multiple boilers are installed, they can be wired together
in a cascade sequence. A maximum of eight boilers can be
controlled from a single control. In this application one boiler
would be designated as the Leader control and all others
would be designated as Member controls.
Once the Leader boiler receives a call for heat from the Enable
input, the 0 - 10 VDC input, or ModBus, the control will
determine what the set point will be. If outdoor air reset is
desired, connect the outdoor air sensor to the terminals on
the Low Voltage Connection Board on the Leader boiler. The
set point will be calculated based on the programmed reset
curve parameters. See the Crest Service Manual to program
the reset curve. If outdoor air reset is not desired, do not
connect the outdoor air sensor. A fixed temperature set point
can be programmed into the control, or provided through the
ModBus interface.
If the water temperature at the controlling sensor is less than
the set point + the turn-off offset - the off-on differential, the
control will initiate a call for heat on the Cascade (see the
Crest Service Manual for an explanation of the offset and
differential). The Leader will energize the lead boiler on the
Cascade. For a new startup this will be the Leader boiler.
The two (2) types of Cascade control are listed below:
Efficiency optimization
Efficiency optimization is chosen when the application is such
that overall heating efficiency is most important. The Cascade
will fire the boilers in a way that is the most efficient. This
allows more boilers to fire at one time, at a lower rate, thus
more efficient. This Cascade is
NOT
allowed in a common
vent application.
Lead/Lag
The Lead/Lag method is chosen when the application requires
firing as few boilers at a time as possible. This method will not
bring on another boiler until the current number of boilers
cannot meet the demand. This Cascade is also
required
when
cascading boilers in a common vent application.
The parameters for both types of Cascade are adjustable.
Reference the Crest Service Manual for a more detailed
description of these adjustments.
Sequence of the cascade
To equalize the run time of all boilers on the Cascade, the
firing sequence will automatically be changed at set intervals.
For the first 24 hours after initializing the Cascade, the
sequence will be changed every hour. After that the sequence
will be changed once every 24 hours.
HWG operation with cascade
For normal mode HWG operation any boiler(s) in the
Cascade can be selected to provide heat for a HWG call.
Select a boiler to be designated as the HWG boiler. Connect
the HWG thermostat or sensor to the terminals on the Low
Voltage Connection Board marked for the corresponding
device. When the boiler receives a HWG call, the Leader
control will take that boiler out of the Cascade sequence. If
another boiler is available, the Leader will start it up to take
its place.
The HWG boiler will adjust its set point to the programmed
HWG boiler set point and will adjust its firing rate to maintain
this. Once the HWG call has been satisfied, the Leader control
will place that boiler back into the Cascade sequence.
Switching of the boiler between HWG operation and SH
operation when there is a call for both does not occur in
Cascade Mode.
When HWG is programmed for Zone Mode, connect the
HWG thermostat or tank sensor to the Leader boiler. When
a HWG call is received, the Leader will modulate the entire
Cascade to bring the system supply temperature up to the
HWG boiler set point (if higher).
Night Setback and Ramp Delay operations with
cascade
Night Setback and Ramp Delay operation of the boilers within
the Cascade are available. Programming will be done through
the Leader boiler. Refer to the Crest Service Manual for
information regarding Night Setback and Ramp Delay.
